A prior art light string is provided with a controller (1) connecting with an electrical wire (3) of the string in order to obtain a flashing effect, as shown in FIG. 1. By use of a circuit in the controller (1), it controls the bulb (2) on the string lighting at different time cyclically. In this design, the controller (1) is connected at an end of the wire (2). The flashing effect is regular without enough variability and it has only a few simply flashing cycles. Moreover, the affixed controller (1) on the light string is not symmetrical and it increases cost and time in manufacture and assembly.